(Note: I posted this using the PlayMaker bug reporter within Unity, but I couldn't attach a repro project afterwards like I thought I would be able to. I'm attaching it here instead. Link to original bug report here:
https://hutonggames.fogbugz.com/default.asp?1349_9ghg44jvi45ft8ct.)
If a custom PlayMaker Action has a reference to a ScriptableObject that has a circular reference with another ScriptableObject, then right-clicking a transition to bring up the context menu will crash Unity. By circular reference, I mean a ScriptableObject that has a reference to another ScriptableObject that, in turn, has a reference to the first ScriptableObject. (It may not be limited to ScriptableObjects, that's just where I noticed it.)
The easiest way to understand this is to simply download and unzip the repro project at this link:
[ADMIN: LINK REMOVED SINCE THE PROJECT INCLUDED PLAYMAKER, PLEASE ALSO REMOVE THE FILE FROM DROPBOX]
Simply open up the Test scene, open the PlayMakerFSM in the PlayMaker Editor, and right-click the only existing transition in the FSM.
Any advice/help would be greatly appreciated. I'm currently needing to resort to hacky workarounds to avoid the circular references. (I have no idea why the right-click context menu should even be exploring references of references in the first place...)